Apple has started to inform macOS users that support for 32-bit apps will be stopped soon. Mac users running macOS 10.13.4 High Sierra will receive a one-time alert when they open a 32-bit app. The Cupertino giant has not informed exactly when support for 32-bit apps on macOS will cease. It had warned developers about the change at Apple's Worldwide Developers Conference (WWDC) 2017, where it noted that High Sierra would be the last version to support 32-bit apps "without compromise." As of now, the alert says that the app is not optimised for a Mac, but the app will continue to work.
The transition to 64-bit hardware and software technology for Mac was started by Apple over 10 years ago, the company said in its support page. It is said to be working with developers to help them transition their apps to 64-bit. It says that the technologies that "define today's Mac experience," for instance, Metal graphics acceleration, work only with 64-bit apps. "To ensure that the apps you purchase are as advanced as the Mac you run them on, all future Mac software will eventually be required to be 64-bit," Apple noted.
To check if an app is 32-bit or 64-bit, go to About This Mac in the Apple menu. Click on the System Report button. Here, from the system report, scroll down to Software, then select Applications. Once you select the individual application, you will see a field titled 64-bit (Intel). If it says Yes, it means the app is 64-bit and a No indicates 32-bit.
Apple says if you have any 32-bit apps, you should check with the software developer to see if a 64-bit version is available.